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          <h1 class="post-title" itemprop="name headline">跨域请求(CORS)要点</h1>
        

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        <p>前端开发的童鞋，应该都有听过跨域请求，但这其中的细节可能还不清楚，比如：</p>
<pre>
* 什么是跨域请求？
* 为什么会存在跨域请求？
* 跨域请求是怎么工作的？
* 如何解决跨域请求？
</pre>

<h3 id="1-什么是跨域请求"><a href="#1-什么是跨域请求" class="headerlink" title="1. 什么是跨域请求"></a>1. 什么是跨域请求</h3><p>跨域请求 - 访问其他域名的资源，随着业务的复杂化及前后端的分离，我们需要经常访问其他域名的资源，因此这里就涉及到跨域访问，下图给出了跨域访问的例子</p>
<p><img src="https://mdn.mozillademos.org/files/14295/CORS_principle.png" alt="image"></p>
<h3 id="2-为什么会存在跨域请求"><a href="#2-为什么会存在跨域请求" class="headerlink" title="2. 为什么会存在跨域请求"></a>2. 为什么会存在跨域请求</h3><p>可能有童鞋就说了，何必这么麻烦，直接允许访问不就行了，也许前期就是这样，于是就出现了CSRF（跨站请求伪造），可以看下图：<br><img src="http://pic002.cnblogs.com/img/hyddd/200904/2009040916453171.jpg" alt="image"></p>
<p>因此网站A就必须添加访问限制，即决策是否允许网站B访问</p>
<h3 id="3-跨域请求是怎么工作的"><a href="#3-跨域请求是怎么工作的" class="headerlink" title="3. 跨域请求是怎么工作的"></a>3. 跨域请求是怎么工作的</h3><p>跨域请求针对不同的请求会采用不用的策略，这里罗列如下：</p>
<h5 id="3-1-简单模式（Simple-requests）"><a href="#3-1-简单模式（Simple-requests）" class="headerlink" title="3.1 简单模式（Simple requests）"></a>3.1 简单模式（Simple requests）</h5><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">请求方法:</span><br><span class="line">	GET</span><br><span class="line">	HEAD</span><br><span class="line">	POST</span><br><span class="line">请求头:</span><br><span class="line">	Accept</span><br><span class="line">	Accept-Language</span><br><span class="line">	Content-Language</span><br><span class="line">	Content-Type</span><br><span class="line">Content-Type:</span><br><span class="line">	application/x-www-form-urlencoded</span><br><span class="line">	multipart/form-data</span><br><span class="line">	text/plain</span><br></pre></td></tr></table></figure>
<p>满足以上特征的请求就是Simple request，采用如下工作模式:</p>
<ul>
<li>浏览器从网站B获取html资源</li>
<li>浏览器发送请求到网站A，并携带网站B的Origin，如Server-b.com</li>
<li>网站A检测自身的Access-Control-Allow-Origin是否包含Origin，如果包含返回正确数据，否则返回空</li>
</ul>
<p><img src="https://mdn.mozillademos.org/files/14293/simple_req.png" alt="image"></p>
<p>PS： Access-Control-Allow-Origin: * 表示允许所有网站方法</p>
<h5 id="3-2-预检模式（Preflighted-requests）"><a href="#3-2-预检模式（Preflighted-requests）" class="headerlink" title="3.2 预检模式（Preflighted requests）"></a>3.2 预检模式（Preflighted requests）</h5><p>除了简单请求外，其他请求访问前需要先发一条预检请求，比如采用OPTIONS，采用如下工作模式:</p>
<ul>
<li>浏览器从网站B获取html资源</li>
<li>浏览器发送OPTIONS请求，并携带如下信息<ul>
<li>Origin - 网站B域名</li>
<li>Access-Control-Request-Method - 待请求方法，如POST</li>
<li>Access-Control-Request-Headers - 待请求头信息</li>
</ul>
</li>
<li>网站A检测自身的Origin、Access-Control-Allow-Methods、Access-Control-Allow-Headers是否包含客户端发送的值</li>
<li>如果包含，则返回200，浏览器发送真正的请求（也需要携带CROS信息），服务器返回正确数据</li>
<li>如果不包含，则返回错误，并返回Origin、Access-Control-Allow-Methods、Access-Control-Allow-Headers</li>
</ul>
<p><img src="https://mdn.mozillademos.org/files/14289/prelight.png" alt="image"></p>
<h3 id="4-常见错误"><a href="#4-常见错误" class="headerlink" title="4. 常见错误"></a>4. 常见错误</h3><ul>
<li>4.1 Access-Control-Allow-Origin不匹配</li>
</ul>
<p>一般会报如下错误：<code>已拦截跨源请求：同源策略禁止读取位于 http://127.0.0.1:19110/uptoken 的远程资源。（原因：CORS 头 &#39;Access-Control-Allow-Origin&#39; 不匹配 &#39;1&#39;）</code></p>
<p>解决方法：检测服务端配置的Access-Control-Allow-Origin，应该包含前端所在服务的域名</p>
<ul>
<li>4.2 Access-Control-Allow-Headers不匹配</li>
</ul>
<p>一般会报如下错误：<code>已拦截跨源请求：同源策略禁止读取位于 http://127.0.0.1:19110/uptoken 的远程资源。（原因：来自 CORS 预检通道的 CORS 头 &#39;Access-Control-Allow-Headers&#39; 的令牌 &#39;if-modified-since&#39; 无效）</code></p>
<p>解决方法：检测服务端配置的Access-Control-Allow-Headers，应该包含前端发送的Access-Control-Request-Headers（可以抓包看前端发送的数据）</p>
<h3 id="更多"><a href="#更多" class="headerlink" title="更多"></a>更多</h3><ol>
<li>跨域是浏览器行为</li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS" target="_blank" rel="noopener">HTTP access control (CORS)</a></li>
<li><a href="http://www.cnblogs.com/hyddd/archive/2009/04/09/1432744.html" target="_blank" rel="noopener">浅谈CSRF攻击方式</a></li>
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